The present invention relates to nuclear reactors and coolant circulation systems therefor.
A nuclear reactor typically includes a core contained within a vessel and a primary cooling system for pumping a primary coolant through the core. The primary coolant typically travels through a fluid circuit wherein the primary coolant receives heat from the core and is cooled externally in a heat exchanger to transfer heat to a working fluid.
If failure of some element of the fluid circuit occurs, as due to a power failure or an external pipe rupture, and circulation of fluid to the core stops, the core may overheat. Because of the hazards associated with such overheating, a reactor may include a secondary or backup cooling system.
It is desirable that a secondary cooling system begin to function immediately upon reduction of flow in the primary cooling system, without reliance on complicated monitoring systems or on operator intervention. One such proposed system is described in a research memorandum by K. Hannerz entitled "Towards Intrinsically Safe Light Water Reactors," Oak Ridge Associated Universities, Institute for Energy Analysis, DE83-017859, July 1983, which is available through the National Technical Information Service. In the reactor described therein, the core is submerged in a pool of relatively cool water, and a primary coolant is circulated through the core and through steam generators by a pumping system. Two horizontal interfaces between stagnant pool water and stagnant primary coolant in communication with flowing primary coolant are provided, one beneath the core and one offset from the top of a riser which extends about 25 meters above the core. Intermixing of the two fluids at the interfaces is limited by their density differences. At each interface, the higher temperature, lower density primary coolant is above the pool water. If the pressure differential in the primary circuit is equal to the static head differential in the secondary fluid, no secondary fluid will flow through the core. However, in the event of reduction of the pressure of the primary coolant at the interface beneath the core as upon failure of the pumping system, water from the pool rises into the core and the core is cooled by natural convection.
A limitation of the above-described system is that, because the static pressure between the two interfaces is essentially equal to the static pressure difference in the pool, head losses in the core must be offset by natural convection to avoid flow from the pool. Thus, the rate of coolant flow is determined by the level of reactivity in the core which occasions such convection, and cannot be varied independently thereof without upsetting the balance of interfaces.